I’m on Reunion, a French overseas territory in the Indian Ocean, and our half-hour flight is the perfect introduction to this mountainous island.

Often overshadowed by its beach-ringed neighbour Mauritius, Reunion is an outdoor enthusiast’s dream, and combining both islands in one holiday means I get to explore two very different worlds.

An island made of mountains

With its jungle interior and volcanic peaks rising to more than 3,000 metres, Reunion is particularly popular with hikers, as many of its natural wonders can only be discovered on foot – or by air.

The mountains near Mafate on Reunion (IRT/PA)

Among them are the three cirques or calderas; tree-covered valley basins hidden deep inland that were created by collapsing volcanoes.

From the air, we spot the most remote of these, Mafate, which is dotted with tiny villages and only accessible via a two-day hike.

Our guide Alexis, a wiry Frenchman with bags of charm, tells us that rafting and canyoning are other popular pastimes here, as is cycling – either along the 200km coastal road or, if you have the stamina, up the steep and winding roads to the island’s interior.

But I decide to skip the high-intensity exploits and instead head south-east to Grand’Anse, where a well-worn forest path leads us to clifftops and uninterrupted ocean views.

Getting to grips with a volcano

The Plage de Grand Anse beach on Reunion (Verena Vogt, PA)

After an hour’s drive further east, we stop at the Grand Brule, a 60-metre-wide hardened lava flow created by repeated outbursts of the Piton de la Fournaise, one of the world’s most active volcanoes.

The grey landscape has an otherworldly feel, and we detect few signs of life as we walk across the rough elephant skin-like surface back to our vehicle.

The eruptions that caused these flows also created lava tunnels; kilometre-long underground networks that can be explored on guided tours.

We follow along a slippery path, descend into a hole no more than two metres wide and find ourselves in a basalt tunnel that stretches for 700 metres up into the mountain, and all the way down to the sea.

Equipped with headlamps, we take a few steps before we switch off the lights and find ourselves in complete darkness.

A nice moment for some quiet reflection – until we hear scuttling noises. Are we being attacked by the white crabs that live in these tunnels? No, it turns out to be our guide playing a joke.

A taste of the tropics

We decide to calm our nerves with food, and as we arrive at the local market in Saint-Paul, we’re reminded that this is a slice of France in the middle of the Indian Ocean, with pain de campagne, macarons and brioche sitting alongside tropical fruit such as goyaviers (small and red with a slightly tangy taste) and combava (a wrinkled type of lime).

Vanilla on sale at a market in Saint-Paul, Reunion (Verena Vogt/PA)

And everywhere we look, there is vanilla. It was on Reunion in 1841 that a young slave discovered vanilla orchids could be pollinated by hand, revolutionising the plant’s cultivation, and there are still more than 100 vanilla producers on the island today.

It’s a bargain, too: A bundle of vanilla pods, vanilla powder and a brightly coloured bamboo vanilla pod container set me back a grand total of €12 (£10).

The Lux Saint Gilles hotel on Reunion (Verena Vogt/PA)

Not surprisingly, the black spice is a key ingredient in Reunion’s cuisine – my culinary highlight is a duck lentil curry with vanilla (around €10/£8) which we enjoy in a small restaurant in the village of Hell-Bourg.

Fish and tropical fruit also feature prominently. The green papaya salad, grilled parrot fish and pineapple carpaccio we’re served on our last evening at our hotel, the colonial-style five-star LUX* Saint Gilles, is the kind of food you’re likely to find all over the island.

Zipping through the jungle in Mauritius

The next day, we say goodbye to Reunion and, after a mere 30-minute flight, land in Mauritius for the second part of our journey.

Best known for its long, pristine beaches, the former British colony immediately lures us into its warm and crystal-clear sea, but there are still plenty of non-water-based activities on offer.

Zip lining at Domaine de L’Etoile, Mauritius (Domaine de L’Etoile/PA)

One afternoon, we head to the Domaine de l’Etoile nature park, which offers one of the world’s longest zip-line courses (£45 per person), with seven lines ranging from 350-750 metres.

For two hours, we zip past fruit bats and through treetops, and once we’re back on the ground, we watch our step to avoid crushing frogs hopping across the wet grass.

We stay at another LUX* hotel, the LUX* Belle Mare, which has recently had a makeover from UK designer Kelly Hoppen, and comes with a distinctly British touch.

The main pool at the Lux Belle Mare hotel on Mauritius (Lux Belle Mare/PA)

A 1933 Rolls-Royce operates as a food stall, fish and chips are served at the beachside bar, and a red phone box allows you to make free calls to anywhere in the world.

The hotel also organises cycling trips, so early one morning we find ourselves on bright red bikes, riding past tobacco fields, along a deserted beach and into the little fishing village of Trou d’Eau Douce, where we’ve just missed the fishermen bringing in the catch of the day.

It’s only 7.30am but the sun is already out in such force that we’re glad to get soaked by roadside sprinklers as we make our way back to the hotel.

We’re drawn back to the sea one last time and end our holiday with a snorkelling trip just off the hotel beach.

I watch a parade of fish swim past my face, and after the adrenaline rush of the past week, an unexpected feeling spreads through my body; a deep sense of serenity.

How to get thereHayes & Jarvis (01293 762 456; hayesandjarvis.co.uk) offers a 10-night twin-centre holiday to Reunion Island and Mauritius from £1,749 per person – a saving of up to £279 per person. The offer includes five nights at the five-star LUX* Saint Gilles in Reunion Island, followed by five nights at the five-star LUX* Belle Mare in Mauritius, both on a B&B basis. The offer includes return international flights from London Heathrow with Air Mauritius. Departs June 3, 2018.